The current trend in satellite communications is to implement multiple beam coverage of congruent narrow spot beams, as it is already the case at Ka-band for current broadband applications. Investigations are on-going to extend the concept to other frequency bands and applications, such as C- and Ku-band.
Multiple beam coverage is known to provide better antenna gain for a given antenna aperture size and significantly increases the communication satellite-based system capacity by frequency spectrum re-use on non-adjacent spot beams. Frequency re-use schemes implemented in satellite-based communication systems use elementary sets or patterns of spot beams, corresponding to the so-called cells commonly used in ground cellular communication networks. Usually a pattern of four spot beams, also referred to as a four-colour scheme, shares the full available spectrum (other patterns including 3 or 7 spot beams may also be considered). The elementary set of spot beams is duplicated or repeated over the entire coverage in such a way that adjacent beams do not use the same combination of carrier frequency and polarisation, so as to minimise the interference between a desired signal within a spot beam and unwanted signals from the adjacent spot beams. The level of interference is usually evaluated with the carrier over interferers ratio (C/I). As an example, a typical four-colour re-use scheme implements frequency and polarisation diversity, i.e. any two adjacent beams within the satellite coverage may either use a different frequency sub-band and/or a different polarisation. The main challenge at antenna level is to produce all the beams with an acceptable cross-over level (typically 3 to 5 dB below the peak gain) in order to ensure high radio frequency (RF) performance over the full coverage.
A conventional reflector antenna configuration wherein feeds are designed to provide proper illumination of the main reflector typically results in poor cross-over level between the beams generated by adjacent feeds (10 dB or more).
This limitation is usually overcome by using 3 or 4 single-feed-per-beam (SFB) single-reflector antennas to produce all the beams in the desired multiple beam coverage. A first configuration, implementing such a solution at Ka-band, that uses eight SFB reflector antennas to produce a dual-band (Tx/Rx) multiple beam coverage is described in the paper of Sudhakar K. Rao, entitled “Parametric Design and Analysis of Multiple-Beam Reflector Antennas for Satellite communications,” IEEE Antennas and Propagation Magazine, Vol. 45, No. 4, pp. 26-34, August 2003. This antenna farm configuration, implemented on the Anik-F2 satellite, comprises four SFB reflectors (Tx) operating in a transmitting mode and four SFB reflectors (Rx) operating in a receiving mode. The reflector apertures have different dimensions in the transmitting mode (Tx) and in the receiving mode (Rx) in order to ensure congruence of the beams and similar cross-over levels regardless of the operating bands. Such a configuration is obviously very restrictive in terms of accommodation within the fairing of the launch vehicle due to the high number of apertures required.
A solution to reduce the number of apertures has been to use dual-band (Tx/Rx) SFB reflector antennas as described in the paper of Sudhakar Rao et al., entitled “Dual-band multiple beam antenna system for satellite communications,” IEEE AP-S International Symposium, Vol. 3A, pp. 359-362, 2005 or as implemented on a few in-flight state-of-the-art commercial satellites as Ka-Sat and Viasat-1. However, using the same reflector aperture at two different frequency bands, corresponding respectively to the transmit (Tx) coverage and the receive (Rx) coverage requires to shape the reflector so as to broaden the receiving beams at the higher frequency band and to ensure that the cross-over level remains similar to the one in the transmit coverage.
Besides, as current beam sizes are in the range of 0.4 to 0.7 degree at Ka-band, reflector apertures in the range of two meters and more are required, which results in a satellite accommodation with two reflector antennas per lateral face and leaves very limited space for other missions.
To further increase the satellite capacity, smaller spot beams are being considered for next generations of High Throughput Satellites (HTS) thus requiring even larger reflector apertures. This constraint combined with the operator's need to allocate more missions on a satellite to increase their revenue calls for antenna farms with a reduced number of apertures while maintaining high level of performance. On-going developments include solutions with a reduced number of apertures to produce a full dual-band multiple beam coverage.
One solution is to use advanced feed systems based on Multiple-Feed-per-Beam (MFB) configurations as described in the paper of Michael Schneider et al., entitled “The multiple spot beam antenna project ‘Medusa’,” 3rd European Conference on Antennas and Propagation (EuCAP), pp. 726-729, 2009. Such a solution requires a focal array with more feeds than beams, typically seven feeds used per beam, with a certain level of overlap between adjacent clusters of feeds to generate proper cross-over between the beams. A Beam Forming Network (BFN) is used to connect a given cluster to its beam port, waveguide technology being usually preferred at Ka-band. However, due to the bandwidth limitations of the BFN, the full coverage needs to be produced with two separate apertures, one aperture for the transmit (Tx) coverage and one aperture for the receive (Rx) coverage.
Other solutions using only one aperture are also proposed.
A first category of solutions as described in the U.S. Pat. No. 7,522,116 B2 uses an over-sized reflector configuration, which may still lead to accommodation issues, or requires the use of advanced and complex reflector technology, e.g. deployable mesh reflectors, for smaller spot beam sizes.
A second category of solutions as for example the multi-beam communication satellite antenna described in the patent application US 2012/0075149 A1 is based on a normal-size reflector configuration but with degraded performance. Such satellite antenna leads to very high spill-over losses in the range of 3 to 10 dB, which significantly affects the antenna gain and overall system performance. These high spill-over losses are related to a poor illumination of the reflector which also produces higher side lobe levels, and as a consequence degraded C/I performance.
U.S. Pat. No. 4,342,036 discloses a broadband communication satellite antenna with a triple band multiple beam coverage that uses only a single main reflector and two sub-reflectors with frequency selective surfaces. The disclosed antenna system does not anticipate specific constraints associated with dual-band (transmit/receive) missions and the optical system is based on a Newtonian model (flat sub-reflector).